A significant source of error in high precision mechanical and electromechanical structures is vibration. Antennas, optical structures, pointing and tracking systems, seeker heads, gravitational and inertial sensors, and guidance platforms can suffer a degradation in performance from external excitation by the local acoustic and vibrational environment.
In the past, the effects of externally induced vibrations in large mechanical structures have been mitigated by such brute-force techniques as stiffening the structure, adding massive mechanical dampers, covering the structure with viscoelastic damping material, or adding acoustic shielding and vibration isolators to shield the structure from the environment.
Another approach to mitigating the aforementioned vibration effects involves electronically damping or controlling the vibrations. In order to accomplish this, an electromechanical transducer, such as a piezoelectric strain transducer, is mounted on the vibrating mechanical structure to provide an electrical signal responsive to the vibrational motion of the structure. This electrical signal may be either applied to a damping resistor connected across the transducer output terminals or fed to electronic processing circuitry for developing an appropriate control signal which is fed back to the mechanical structure by means of another electromechanical transducer. Regardless of the particular approach employed, the inherent capacitance of the signal-extracting transducer significantly limits the degree of coupling to the external electronic circuitry. This, in turn, limits the degree of electronic damping or control which may be achieved.
In the co-pending application of R. L. Forward, Ser. No. 901,550 filed on May 1, 1978 now abandoned, an arrangement is disclosed for tuning out the inherent shunt capacitance of the transducer by connecting across the transducer output an inductor which provides the appropriate inductance to resonate with the transducer capacitance. Such an arrangement is highly effective in eliminating the effect of the transducer capacitance at vibration frequencies in the vicinity of the inductance-capacitance resonant frequency, although its effectiveness is reduced as the vibration frequency departs from the resonant frequency.
Another technique for compensating for such transducer capacitance is disclosed in the co-pending application of R. L. Forward, Ser. No. 904,169, filed May 8, 1978 now U.S. Pat. No. 4,158,787, and also assigned to the present assignee. In this latter application, a negative capacitance circuit is placed across the transducer to provide broadband cancellation of the shunting effect of the transducer capacitance. With the inherent transducer shunting capacitance thus cancelled, substantially all of the transducer current may be coupled through a damping resistor placed across the transducer. By this means, high levels of damping can be achieved for many vibrational modes simultaneously.
It is a general object of the present invention to minimize the effects of vibrations in mechanical and electromechanical structures.
It is another object of the present invention to reduce the effects of inherent transducer capacitance in a transducer-coupled electromechanical system over a wide range of vibrational frequencies.
It is a further object of the present invention to provide improved feedback damping utilizing circuits which simulate low-noise temperature resistances.